The invention relates to a method of eliminating disturbances from fixed error sources in the video signal from an infrared (IR) camera of the pyroelectric vidicon type and a device for carrying out the method.
In an IR camera of the pyroelectric vidicon type a heat picture is produced upon a pyroelectric detector plate. This picture is read by means of a thin electron beam scanning the detector plate in the same manner as in a common TV camera tube, usually once every fiftieth of a second. A video signal representing the heat picture on the detector is obtained as an output signal. However, the video signal is not a measure of the absolute value of the temperature at each point of the plate but it is only a measure of the temperature variations with time. The pyroelectric vidicon therefore does not "see" stationary pictures but only variable pictures. In order to be able to see a stationary picture (i.e. to convert the stationary picture to a variable picture) there are two methods available. One method, which is called "chopping", involves alternately opening and closing a mechanical aperture or the like so that during every other frame the heat picture is projected on the detector plate, while during the intermediate frames the heat picture is not projected on the detector plate. The second method of converting a stationary picture to a variable picture which can be observed by means of a pyroelectric vidicon is to sweep the whole IR camera. This so called "panning" assures that new sections of the heat picture are continually projected on the detector plate. In the operation of such a camera each portion of the heat picture or image is represented by a spot on the detector and segment of the video signal. When sampling the video signal it is suitable to choose the sampling frequency so that each image portion comprises only one single sample of the video signal. A video signal picture frame (i.e. that part of the video signal which corresponds to a complete scanning of the heat picture) therefore will consist of some "spikes" in a signal which is otherwise zero (or has a reference value), or a number of samples differing from zero while remaining samples are equal to zero. Each such "spike" or "sample" different from zero represents an image portion.
But besides signal components in the video signal which represent the image, there are also error signals or disturbance signals, for example caused by internal errors in the camera such as inhomogeneities in the detector plate. These disturbances are assumed in the present case to be fixed (i.e. constant) from one picture scanning to the next picture scanning.